WASHINGTON (USA TODAY) -- Attorney General Eric Holder acknowledged Wednesday that four U.S. citizens had been killed in counter-terrorism drone strikes since 2009.

In a letter to Senate Judiciary Committee Chairman Patrick Leahy, Holder said only one of the four, radical cleric Anwar al-Awlaki, the inspiration for several plots against the U.S., was "specifically targeted'' in counter-terrorism operations against al-Qaeda.

Three others, including al-Qaeda propagandist Samir Khan, Al-Awlaki's son, Abd al-Rahman, and Jude Kenan Mohammed were killed during the same time period but "these individuals were not specifically targeted by the United States.''

Holder said the letter, directed by President Obama, is in response to congressional inquiries about the "administration use of lethal force against U.S. citizens.''

White House officials emphasized this passage from the letter: "Indeed, the Administration informed the relevant congressional oversight committees that it had approved the use of lethal force against Al-Awlaki in February 2010 -- well over a year before the operation in question."

The five-page communication comes in advance of Thursday's scheduled national security speech by President Obama and his attempts to address one of the most volatile issues in the nation's counter-terrorism strategy.

Drones are a major topic for Obama's speech, to be delivered at the National Defense University in Washington .

"Based on generations-old legal principles and Supreme Court decisions handed down during World War II," Holder said in the letter to Leahy, "as well as during the current conflict, it is clear and logical that United States citizenship alone does not make such individuals immune from being targeted.''

"Rather, it means that the government must take special care and take into account all relevant constitutional considerations, the laws of war and other law with respect to U.S. citizens - even those who are leading efforts to kill their fellow, innocent Americans.'' The letter also was transmitted to other congressional leaders.

Outlining the administration's reasoning for the use of lethal force, Holder said such targets are U.S. citizens affiliated as senior al-Qaeda operatives in foreign countries who the government has determined pose an "imminent threat of violent attack against the United States'' and whose capture is not "feasible.'' The conditions also call for the operations to be "conducted in a manner consistent with applicable law of war principles.''

Al-Awlaki, as an operational leader of al-Qaeda's affiliate in the Arabian Peninsula that is blamed for the attempted 2009 Christmas Day bombing of an airliner over Detroit and other plots, "satisfied all conditions'' for his targeted killing in September 2011.

"In his role, al-Awlaki repeatedly made clear his intent to attack U.S. persons and his hope that these attacks would take American lives,'' Holder said.

Human rights advocates, who have been vocal in their criticisms of the administration's use of such tactics, were not satisfied with Holder's explanation.

"No one should be reassured by Attorney General Holder's letter,'' said Zeke Johnson, Amnesty International USA's director of Security with Human Rights. The Obama administration continues to claim authority to kill virtually anyone anywhere in the world under the 'global battlefield' legal theory and a radical redefinition of the concept of imminence.''

Dixon Osburn, director of Human Rights First's Law and Security Program, said the group was "deeply concerned that the administration appears to be institutionalizing a problematic targeted killing policy without public debate.''

"The American public deserves to know whether the administration is complying with the law, and Congress should debate the legal and policy implications of our targeted killing operations,'' Osburn said.

Obama pledged to be more open his counter-terrorism plans during his Feb. 12 State of the Union address.

That night, Obama said that "in the months ahead, I will continue to engage with Congress to ensure not only that our targeting, detention and prosecution of terrorists remains consistent with our laws and system of checks and balances, but that our efforts are even more transparent to the American people and to the world."

In his speech Thursday, Obama is also scheduled to discuss efforts to close the terrorism prison at Guantanamo Bay, Cuba, in part by transferring some detainees to other countries. White House spokesman Jay Carney said Obama will outline "structures" for succeeding administrations, "so that in the carrying out of counter-terrorism policy, procedures are followed that allow it to be conducted in a way that ensures that we're keeping with our traditions and our laws."